English forms in sn- might be due to a misreading of the uncommon fn- (represented in only eight words in Clark Hall, mostly in words to do with breathing), or from Norse influence. OED suggests Middle English fnese had been reduced to simple nese by early 15c., and sneeze is a "strengthened form" of this, "assisted by its phonetic appropriateness." Related: Sneezed; sneezing. To sneeze at "to regard as of little value" (usually with negative) is attested from 1806.

sneeze (snēz)v.sneezed, sneez·ing, sneez·es To expel air forcibly from the mouth and nose in an explosive, spasmodic involuntary action resulting chiefly from irritation of the nasal mucous membrane. n. The act or an instance of sneezing.